If Adobe weren't so obsessed with releasing small incremental upgrades every year they might sell more copies.
Creative Suite is a big investment. The paid for jump from CS5 to CS5.5 wasn't huge but the upgrade price was. Many people just hang back and keep on using old version (often unaware that Adobe charge even more for an upgrade if you don't have the previous version).
Ideally they'd like us all on the subscription model but I can see serious user resistance especially in the video industry. People like to get an edit setup that works and stick with it rather than upgrading every 12 months.